The subject invention relates to an internal combustion engine of two-stroke type, mainly intended for a working tool, preferably a chain saw or a trimmer, and provided with a muffler device.
For working tools run by internal combustion engines generally two-stroke engines are used, mainly due to their low weight and simple design. Also, the crankcase scavenging enables a lubrication system independent of position, in which the engine is lubricated by oil that is added to the air/fuel mixture scavenged through the crankcase. The all-position lubrication system is necessary for typical working tools, e.g., chain saws, since they are to be used in a lot of different working positions.
Two-stroke engines for mopeds and motorcycles generally have a so-called tuned exhaust system. Reflected pressure pulses from the exhaust system will press scavenging gases back into the cylinder so that the engine""s scavenging losses are reduced. In total this means that both the power output and fuel consumption can be improved in comparison with a non-tuned exhaust system. However, in order to function well, the tuned exhaust system requires large lengths of pipe as well as large cross-section areas in the exhaust duct. Such a muffler for a chain saw would be at least half a meter long and consist of a first conically expanding duct section by approximately 8 degrees, and a second conically narrowing section by approximately 12 degrees. Between these conical parts there should be located a straight part having a diameter, or actually a cross-section area, which is many times larger than the exhaust port. If, for example, the exhaust port would have a diameter of 30 mm, then the straight part would have a diameter of approximately 60-100 mm and approximately a 10 times larger cross-section area. Subsequent to the pipe, an absorption muffler should be connected in order suppress sound to a reasonable level.
As mentioned above, such a tuned exhaust system is based on reflected pressure pulses as well as a low total pressure drop. Regarding working tools, it has been determined that such a tuned exhaust system will be far too large and heavy, even if the pipe system is provided with a lot of curves. This is due to the large cross-section areas. A working tool must be very lightweight, compact and handy in order to serve its purpose. Consequently, tuned exhaust systems are normally not used for working tools. Instead they have lightweight and compact mufflers in which the sound mainly is damped by throttling in the muffler. A larger cylinder volume is used to reach the preferable effect. Owing to the fact that there are great differences between the layout of two-stroke engines with tuned exhaust systems and without tuned exhaust systems, it is difficult to transfer experiences from one field to the other.
A well-known problem of two-stroke engines is their relatively high fuel consumption caused by high-scavenging losses, i.e., scavenging gases which flow straight out into the exhaust system. This also results in high emissions, especially of hydrocarbons. As mentioned above, the difficulties to overcome this problem are especially big for two-stroke engines with lightweight and compact mufflers, i.e., with non-tuned exhaust systems. The high emission of hydrocarbons also results in certain problems when using a muffler with catalytic conversion. For example, the very high energy content of the exhaust gases leads to a very high heat generation in the catalytic converter as well as in the surrounding muffler. This high extent of heat generation could mean that the conversion ratio in the catalytic converter must be kept down. Consequently, the high scavenging losses increase the fuel consumption at the same time as they complicate a co-operation with an exhaust catalytic converter.
The purpose of the subject invention is to substantially reduce the above outlined problems for a two-stroke internal combustion engine that is provided with a silencing device.
The above-mentioned purpose is achieved in a device in accordance with the invention, having the characteristics appearing from the appended claims.
The two-stroke internal combustion engine in accordance with the invention is thus essentially characterized in that a pipe, which is either straight or bent, and having an adapted or tuned length and a closed outer end, is arranged in connection to a cylinder exhaust port of the engine, as well as an outlet, which lead the exhaust gases to the surrounding air. When the exhaust port is opened a powerful pressure pulse is created which runs into the pipe and is reflected by its closed outer end. If the pipe is given a correctly tuned length for a given rotational speed, for example maximum power rpm, the reflecting pressure pulse from the pipe will increase the exhaust pressure outside the exhaust port before it will be closed, so that the pressure becomes higher than the pressure inside the cylinder. Thereby further scavenging gases are prevented to flow out and some of the scavenging gases could even be pressed back into the cylinder. Therefore the scavenging losses are reduced and the engine""s power can be increased and its specific fuel consumption be reduced.
The exhaust emissions, especially of hydrocarbons, will be reduced substantially, with the result that also the heat strain on an eventual following exhaust catalyser will be reduced, so that its co-operation with the engine will be simplified.
Although the invention primarily is intended for an engine for a working tool the invention can be used for other kind of motor applications.
Further characteristics and advantages of the invention will become more apparent from the detailed description of preferred embodiments and with support of the drawing figures.